Immutable.js would be great if it were actually maintained. FB hasn't made or approved a real pull request since April, and there are hundreds of pending issues that haven't been addressed or even responded to in over a year.
I went through the pull requests and checked 5 or so that were open for a long time, all of them were glaringly missing something.
Some do not pass tests, some have obvious styling issues, some people haven't signed the contributor agreement, some offer big changes with very little reasoning behind it.
I also prefer projects that are more explicit about the reason something is rejected and answer RTFM-type issues, but this is not a requirement to be a successful project.
Now neither of them are really maintained.
For a lot of usages, something like Ramda or Lodash/FP will do the trick (Ramda lenses or Lodash/FP's get and set methods work great to handle immutable data structures), but if you really wanted structural sharing and stuff, now you're kindda stuck.
Instead I ended up just deepFreezing my data (only in development) and wrote a bunch of pure functions which perform all the important array/object functions but return a new array/object instead of mutating. It was a really great exercise in functional programming and I'd recommend it to anyone wanting immutable data and to understand immutable programming styles.
They are all just a few lines, so they make for great practice to write yourself.